The desire to maintain a youthful appearance by preventing or reducing wrinkles in the skin is an important issue in human society. Many techniques have been designed to achieve the above issue. One of the techniques, e.g., known from the published international patent application WO 2008/001284 A2, is to create a focal spot in a dermis layer of the skin to be treated. Said WO application discloses a skin treatment device with a laser source and focusing optics. The device emits a laser beam. The power of the laser is selected such that Laser Induced Optical Breakdown (LIOB) affects the skin in order to stimulate re-growth of skin tissue and reduce wrinkles. This LIOB is based on strong non-linear absorption of the laser light by the skin tissue, which occurs above a certain threshold value for the power density of the laser light.
In addition to LIOB, also other light-based treatment processes are known, such as fractional photothermolysis to create microscopic thermal lesions in the epidermis or dermis of the skin tissue to stimulate re-growth of the damaged skin tissue to rejuvenate the skin tissue and reduce wrinkles Rejuvenation processes through light-based treatment inside the skin tissue (such as said laser induced optical breakdown and fractional photothermolysis) require high photon densities in localized regions in the tissue. When light propagates through the skin tissue, absorption, scattering and deformation of wave fronts often result in a relatively broad distribution of light at the focus position and a distribution of the light energy over a relatively broad focal spot. Because of the high intensity requirement and the spreading of the light over a relatively large focal spot, the overall intensity of the treatment light necessary to perform the light-based treatment of skin tissue is increased. This typically increases the requirements to be met by the light sources used in such skin treatment devices, causing said skin treatment devices to be more expensive, and the increased overall intensity of the treatment light increases possible damage to skin tissue surrounding the treatment position, which is also undesirable.
Consequently, in the known skin treatment devices the quality of focusing the treatment light from the light source to the focus position is not optimal, which requires the known skin treatment devices to use an excess of light energy to produce this light-based treatment process.